Free cutting alloys



Patented Apr. 13, 1937 FRE ALLOYS Louis W; pf and Walter A. Dean, Cleveland, Ohio, 'assignors to Aluminum Company of America, Pennsylvania No Drawing. 7 Application December 28, 1935,. Serial No. 56.551 v v Pittsburgh, Pa., 1 a corporation of Claims. (01. 75-141);

The invention relates to aluminum base alloys and is particularlyfconcerned with aluminum base alloys containing magnesium, silicon and zinc.

5 Aluminum base alloys containing from about 0.1 per cent to 1.0 per cent ofv magnesium, and from about 0.5v per cent to about 2.5 per cent of silicon have heretofore been used. They are usuallyin the wrought condition since they are 10 readily susceptible to mechanical deformation such as rolling, forging, or extrusion. It is possible, by appropriate thermal treatments to very substantially improve their physical properties. After a high temperature solution treatment they do not age-harden to any great extent at room temperature but may be artificially aged by temperatures somewhat in excess of room temperature as is well known in the art. For this reason these alloys may be wrought to substantially 0 final form, then subjected to a high temperature solution treatment, and while in a relatively soft condition they may be given certain final working steps at low temperature. Since they do not materially age at room temperature these finishing steps may be performed at any convenient time. They may ultimately be artificially aged if the maximum tensile strength, yield strength, and hardness are desired.

The strength of the aluminum-magnesiumsilicon alloys, and to some extent their casting qualities, may be further improved by the addition of 0.1 to 3 per cent of zinc. Because of the high strength which is attainablein these alloys when heat treated and aged, they are generally used in this condition. Generally speaking, the zinc increases the tensile strength, hardness and yield strength without unfavorably affecting the ductility or corrosion resistance.

Since all commercial aluminum base alloys contain some silicon as an impurity, usually less than, or not greatly in excess of 0.5 per cent, it is to be understood that the amount of silicon discussed herein, and hereinafter claimed, represents the total quantity of silicon present, and

that the silicon content of the aluminum used should be known in order to provide a basis for determining the eventual composition of the aluminum-magnesium-silicon-zinc alloy.

There are, however, some applications wherein aluminum-magnesium-silicon-zinc alloys as hereinabove disclosed might be conveniently and profitably used except for an inherent disadvantage which militates against their use in the production of certain articles requiring exacting machining operations. Mechanical cutting operations such as drilling, shapingiorlathe-cutting are successfully carried outonly by" using certain precautions which increase ,the'cost'of production and perhaps favor the choice of another metal or..

alloy which can be machined more readily but which is 'not so desirable in other respects, as

for example, in physicalproperties. When alloys,

are difficult to machine this disadvantage becomes evident, in many cases, through rapid wear of'the cutting tool edge, so that frequent tool re-sharpening is required. Despite continual lubrication the machine surface is rough and iiregular, and the chip has a tendency to form a continuous curl or spiral which often fouls thetool or the moving parts of the machine. It is immediately apparent that there is need for an alloy of good working characteristics and. satisfactory physical properties, yet possessing such 3 favorable machining properties that the complete machining operations may be performed economically and successfully, and'may be productive of a pleasing surface appearance.

Accordingly an object of our invention is the provision of an aluminum base alloy containing from about 0.1 to about 1.5 percent of magnesium, from about 0.5 to 2.5 per cent of silicon and from about 0.1 to 3.0 per cent of zinc, which may be readily and economically machined.

Our invention resides in the discovery that the foregoing object is effected by the addition of two or more of the elements lead, tin, thallium, cadmium or bismuth. The aluminum-magnesiumsilicon-zinc alloys to which these elements are added in the proportions specified below, are known as free cutting or free machining alloys l-ecause they can be machined more rapidly thansimilar alloys without these elementsand yet have as good or a better finished surface. After an extended series of investigations we have discovered that these five metals when added to aluminum-magnesium-silicon zinc alloys, form a class of alloying elements by reason of their favorable effect upon the machining properties of these. alloys. In recognition of this effect we term lead,tin, thallium, cadmium and bismuth free machining elements. We have further discovered that the simultaneous presence of two or more of these elements is productive of an improvement in free machining characteristics which is considerablygreater than that caused by the presence of the same total amount of a single free machining element. For example, the addition of 0.5 per cent of lead and 0.5 per cent of bismuth to an aluminum base alloy containing about 0.5 per cent of magnesium, 1.0 per cent of intervention: ofso-called rich alloys. 1 a matter-oi fact,- we have observed fthat of all the metals whose melting pointiswlower than about ments' have selected arevtheonlyiones.=-which commercially suitable: and which imparti'ree cutting characteristics a but, do not i -havq'e an un- ,1 1e 1 1 effect on; the fundamental physical propertiesbi the base alloy.1; 1 The-itotal amount :0: free-machining elements should inot'zbe: less; than about 0.05- per; cent 1 since .below tthis amount there ti'sscarcely; any. advaneflecti {We have determined that amaxlimit; 01 about; fir pe'r c'ent-- total of k-two: or moreoffthe 'i'reeimach'ining elements :iswsuflicient forr'vsatisfactory commercial results; since:v althoughi'ithe-lfree machining eflectpersists beyond this: amount, certain of 1 the other physical properties maybeuniavorably affected- .1 r z j Aluminum-magnesium-silicon-zinc alloys containing" two. or more or. the free machining elements, lead, .tin tha'llium, cadmium'and bismuth may beemachined more rapidly, with less. tool wear,lesstoolsharpening, better qualityoi chip and betterv machined suriacethanthesame base alloys without the free machining additions, and in tact better than the same base alloys containing an equivalent total amount of a single free 40 machining element. I

l a-preferred alloy within the range of the respective elements disclosed hereinabove we suggest an alloy containing about 0.6 percent of magnesium, 1.0 per cent 01' silicon, 1.5 per cent 01' zinc, and a total of 3 per cent of free machining elements, the balance being aluminum. For some applications whichdo not require a high degree of tree machining quality, a lesser amount of the free machining constituents may be added to the alloy base, for instance 0.5 per cent lead and-0.5 per cent bismuth, or 0.5 per cent bismuth and'0i5 per cent cadmium.

For certain purposes, notably the improvement of tensile strength, hardness and grain structure, the alloys as hereinabove disclosed may be improved by the addition of one or more of the group of elements composed of' molybdenum, vanadium, titanium, tungsten, zirconium and chromium. From 0.05 to 1 per cent of any one of these elements may be used alone, but if more than one is employed the total amount should not exceed about 2 per cent.

It is characteristic of the five elements, lead, tin, thallium. cadmium and bismuth that they *form with aluminum 9. series of alloys of limited liquid solubility. We have reason to believe that the free machining elements are the only ele'- ments which exhibit this characteristic, with the possible exception of several metals which are 70 not regarded as having any commercial promise as additions to aluminum base alloys. Within the range disclosed and claimed however the free machining elements may be added without unusual difliculty. We suspect that this characterthermal treatments well known in: the to im'- prove their strength "andthard'ness. "We have iound that asolution heat treatment and-subsequent does, not impair the free machining quality of the alloys and in many instancesthe treatment even-i-'tends.;to improve thisproperty.

. mummy purposessal relatively'highstrengthand hardness are? necessary-, zto'a the ssuccesstni 1 per; i'ormance ofgth'e machined article and hence the alloy. must we L heat spa; treatment is generally! As hereinabove indicated the free machining elements, reason of' atheir; melting point, may ,be added :sto themolten aluminum-impure metallic @rdrm. owever, mncexsome; -dimculty may be encountered in introducing sthem 'iifthe higher percentages or. .our 1* disclosed .-range -?we prefer to 'usethemethod which is" more fully described in U. s. Patent No. rpsaozs, issued March 15, 1934. Briefly it involvesheatingwthe i melt to a somewhat "higher temperature than is customary, and' vigorously-wstirring-it in excess-of a critical periodoftime. a

The term "aluminum". as used herein and in the appended claims. embraces theusual impurities found in aluminum ingot of commercial grade or picked up in the course of the ordinary handling operations incidentto melting practice.

We claim:

pplied prior-to he machining? opera+ 1. An aluminum base alloy consisting of about 0.6 per cent magnesium, 1.0 per cent.silicon,.1.5 per cent zinc, 0.5 per cent bismuth and 0.5 per cent cadmium, the balance being. aluminum.

2. An aluminum base alloy consisting of about 0.6 per cent magnesium, 1.0 per cent silicon, 1.5 per cent zinc, 0.5 per cent lead and 0.5 per cent bismuth, the balance being aluminum.

3. A free cutting'alloy containing from 0.1 to 1.5 per cent of magnesium, from 0.1.to 3 per cent of zinc, 0.5 to 2.5 per cent of silicon, and a total of from 0.05 to 6 per cent of at least two of the elements from the following metals, lead, tin, thallium, cadmium, and'bismuth, to improve its machining properties, the balance being substantiallyaluminum.

4. A tree cutting alloy containing-from 0.1 to

, 1.5 per cent of magnesium, from 0.1 to 3 per cent of zinc, from 0.5 to 2.5 per cent of silicon, from 0.05 to 2 per cent of hardening metal from the group composed of molybdenum, vanadium, titanium, tungsten, zirconium and chromium, and a total of from 0.05 to 6 per cent 01' at least two of the elements from the following metals, lead,

tin, thallium, cadmium, and bismuth, to improve its machining properties, the balance being sub- 

